Intra-individual factors affecting lipid, lipoprotein and apolipoprotein measurement: a review

Functionalisation of brush polyethylene glycol polymers with specific lipids extends their elimination half-life through association with natural lipid trafficking pathways

Polymeric prodrugs have been applied to control the delivery of various types of therapeutics. Similarly, conjugation of peptide therapeutics to lipids has been used to prolong systemic exposure. Here, we extend on these two approaches by conjugating brush polyethylene glycol (PEG) polymers with different lipid components including short-chain (1C2) or medium-chain (1C12) monoalkyl hydrocarbon tails, cholesterol (Cho), and diacylglycerols composed of two medium-chain (2C12) or long-chain (2C18) fatty acids. We uniquely evaluate the integration of these lipid-polymers into endogenous lipid trafficking pathways (albumin and lipoproteins) and the impact of lipid conjugation on plasma pharmacokinetics after intravenous (IV) and subcutaneous (SC) dosing to cannulated rats. The IV and SC elimination half-lives of Cho-PEG (13 and 22 h, respectively), 2C12-PEG (11 and 17 h, respectively) and 2C18-PEG (12 h for both) were prolonged compared to 1C2-PEG (3 h for both) and 1C12-PEG (4 h for both). Interestingly, 1C2-PEG and 1C12-PEG had higher SC bioavailability (40 % and 52 %, respectively) compared to Cho-PEG, 2C12-PEG and 2C18-PEG (25 %, 24 % and 23 %, respectively). These differences in pharmacokinetics may be explained by the different association patterns of the polymers with rat serum albumin (RSA), bovine serum albumin (BSA) and lipoproteins. For example, in pooled plasma (from IV pharmacokinetic studies), 2C18-PEG had the highest recovery in the high-density lipoprotein (HDL) fraction. In conclusion, the pharmacokinetics of brush PEG polymers can be tuned via conjugation with different lipids, which can be utilised to tune the elimination half-life, biodistribution and effect of therapeutics for a range of medical applications. STATEMENT OF SIGNIFICANCE: Lipidation of therapeutics such as peptides has been employed to extend their plasma half-life by promoting binding to serum albumin, providing protection against rapid clearance. Here we design and evaluate innovative biomaterials consisting of brush polyethylene glycol polymers conjugated with different lipids. Importantly, we show for the first time that lipidated polymeric materials associate with endogenous lipoprotein trafficking pathways and this, in addition to albumin binding, controls their plasma pharmacokinetics. We find that conjugation to dialkyl lipids and cholesterol leads to higher association with lipid trafficking pathways, and more sustained plasma exposure, compared to conjugation to short and monoalkyl lipids. Our lipidated polymers can thus be utilised as delivery platforms to tune the plasma half-life of various pharmaceuticals.

The challenging use and interpretation of blood biomarkers of exposure related to lipophilic endocrine disrupting chemicals in environmental health studies

The use of exposure biomarkers has been growing during the last decades, being considered the 'gold-standard' approach for individual exposure assessment to environmental chemicals. However, lipophilic endocrine disrupting chemicals (LEDC) have specific physicochemical and biological properties implying particular analytical challenges and interpretative caveats. The epidemiological literature is therefore afflicted by methodological inconsistencies and results divergences, in part due to recognised sources of exposure measurement error and misinterpretation of results. The aim of the present review is to identify external and endogenous sources of variability and uncertainty associated with the LEDC blood biomarkers in epidemiological studies. The dynamic nature of blood and an overview of the known mechanisms of transport, storage and partition of LEDCs in the organism are first described. The external sources of variability and uncertainty introduced at pre-analytical and analytical level are subsequently presented. Subsequently, we present some specific cases where the dynamics of lipids and LEDCs may be substantially modified and thus, the interpretation of biomarkers can be particularly challenging. The environmental obesogens as source of biomarkers variability is also discussed in the light of the most recent findings. Finally, different modelling approaches (statistical and pharmacokinetic models) proposed to improve the use and interpretation of biomarkers are appraised.

Free Fatty Acids as an Indicator of the Nonfasted State in Children

BACKGROUND

Ensuring children are fasting for blood draws is necessary to diagnose abnormalities in glucose homeostasis. We sought to determine if serum free fatty acid (FFA) concentrations might be a useful marker to differentiate the fed and fasted states among children.

METHODS

A total of 442 inpatient (fasting) and 323 (postglucose load) oral glucose tolerance test samples of glucose, insulin, and FFA from children (age 5-18 years) who had healthy weight, overweight, or obesity were examined by receiver operating characteristic (ROC) curve analysis to identify a cut point for nonfasting. In a cross-sectional study, we compared mean FFA and percentage of FFA values below this cut point as a function of inpatient (n = 442) versus outpatient (n = 442) setting.

RESULTS

The area under the curve of FFA was significantly better (P values < .001) than the area under the curve of glucose or insulin for identifying nonfasting. FFA <287 mEq/mL had 99.0% sensitivity and 98.0% specificity for nonfasting. Mean FFA was lower in outpatients than inpatients (P < .001); only 1.6% inpatient but 9.7% outpatient FFA values were consistent with nonfasting (P < .001).

CONCLUSIONS

Clinicians cannot assume that pediatric patients are adequately fasted on arrival for fasting blood work. On the basis of having significantly lower outpatient than inpatient FFA values and more frequently suppressed FFA, children appeared less likely to be fasting at outpatient appointments. FFA value <287 mEq/mL was a sensitive and specific cutoff for nonfasting in children that may prove clinically useful.

Detection of Lipid and Amphiphilic Biomarkers for Disease Diagnostics

Rapid diagnosis is crucial to effectively treating any disease. Biological markers, or biomarkers, have been widely used to diagnose a variety of infectious and non-infectious diseases. The detection of biomarkers in patient samples can also provide valuable information regarding progression and prognosis. Interestingly, many such biomarkers are composed of lipids, and are amphiphilic in biochemistry, which leads them to be often sequestered by host carriers. Such sequestration enhances the difficulty of developing sensitive and accurate sensors for these targets. Many of the physiologically relevant molecules involved in pathogenesis and disease are indeed amphiphilic. This chemical property is likely essential for their biological function, but also makes them challenging to detect and quantify in vitro. In order to understand pathogenesis and disease progression while developing effective diagnostics, it is important to account for the biochemistry of lipid and amphiphilic biomarkers when creating novel techniques for the quantitative measurement of these targets. Here, we review techniques and methods used to detect lipid and amphiphilic biomarkers associated with disease, as well as their feasibility for use as diagnostic targets, highlighting the significance of their biochemical properties in the design and execution of laboratory and diagnostic strategies. The biochemistry of biological molecules is clearly relevant to their physiological function, and calling out the need for consideration of this feature in their study, and use as vaccine, diagnostic and therapeutic targets is the overarching motivation for this review.

Systematic error of serum triglyceride measurements during three decades and the effect of fasting on serum triglycerides in population studies

BACKGROUND

An uncontrolled systematic error in serum biomarkers may be a serious problem when comparing their trends both within and between populations. The aim of the study was to assess which factors are responsible for systematic errors in the measurement of serum triglycerides (Tg) and the effect of fasting on serum triglycerides in Finnish population surveys.

METHODS

Data on precision and accuracy during 30 years for serum triglycerides were documented from participation in 492 rounds of five different external quality assessment (EQA) programs. Data on fasting and health status from questionnaires were combined from three population surveys comprising 27,131 participants.

RESULTS

The mean annual accuracy (bias) of the Tg methods from all EQAs during 1978-2007 was -1.54% (95% CI -2.25, -0.83). The mean relative change in triglyceride concentration per fasting hour was -3.7% (95% CI -4.2, -3.1) in all subjects. A minimum serum Tg concentration was seen in men and women who had fasted for at least 8 and 7 h, respectively.

CONCLUSIONS

The mean bias in serum Tg analyses has been very small throughout the 30-year period. Fasting has a considerable effect on triglyceride levels, but they can be converted either to fasting or non-fasting levels using specific factors.

Atomic Force Microscopy: Interaction Forces Measured in Phospholipid Monolayers, Bilayers and Cell Membranes

Atomic force microscopy (AFM) is a powerful technique which is commonly used to image surfaces at the nanoscale and single-molecule level, as well as to investigate physical properties of the sample surface using a technique known as force spectroscopy. In this chapter, we review our recent research where we used AFM to investigate physical properties of phospholipid monolayers, bilayers, and cell membranes. We describe the experimental procedures for AFM imaging, force measurements, and theoretical models to analyze force spectroscopy data. The data obtained allowed correlations between AFM topography and local adhesion and mechanoelastic properties of supported lipid bilayers in water, supported pulmonary surfactant films in air, and the plasma membrane of epithelial type II cells. Finally, AFM was applied to help elucidate the effect of anesthetics and cholesterol present in the lipid films.

[Apolipoprotein A-I and B distribution among the employees and their partners in Korea]

OBJECTIVES

This study was performed to investigate the distribution of apolipoproteins A-I and B among Korean employees and their partners.

METHODS

The study population consisted of 7,633 men and women (4,578 men and 3,054 women) residing in Seoul and Kyung-gee Do, with an average age of 43.5 +/- 8.3 years. Blood samples were collected following at least 12 hours of fasting. Apolipoproteins A-I and B were measured using a Behring Nephelometer analyzer. The body mass index (BMI) for each participant was calculated as weight (kg) divided by height squared (m2). Information on health-related behaviors such as exercise, alcohol intake, and smoking habits was collected through self-administrated questionnaires.

RESULTS

The mean concentrations of Apo A-I were 132.6 +/- 22.3 mg/dL and 142.9 +/- 24.8 mg/dL in the men and women, respectively. The concentration of Apo A-I increased significantly across all age categories of men. The mean concentrations of Apo B were 101.7 +/- 23.2 mg/dL and 87.8 +/- 23.5 mg/dL in the men and women, respectively, and Apo B increased significantly across all age categories for both the men and women. Exercise and BMI were major determinants for Apo A-I and B levels. The 10th percentile of Apo A-I concentration was 109 mg/dL in the men and 113 mg/dL in the women, and the 90th percentile of Apo B concentration was 131 mg/dL in the men and 118 mg/dL women.

CONCLUSIONS

For the prevention of coronary artery disease, we recommend that for individuals in the 10th percentile of concentration for Apo A-I and the 90th percentile of concentration for Apo B, active preventive interventions such as weight loss and exercise should be taken. This study, within its limitations, may be useful for evaluating apolipoprotein A-I and B concentrations in Korean adults.

Shift working and risk of lipid disorders: a cross-sectional study

BACKGROUND

previous studies have indicated on association between shift work and lipid profile disturbances. Lipid profile disturbances could be due to internal desynchronization. The aim of this study was to analyze whether there is relationship between shift work and serum lipids, fasting blood glucose and hypertension.

RESULTS

A total of 424 rail road workers between the ages of 21 and 64 years in this study filled out a questionnaire, and total cholesterol, triglyceride and HDL-C concentration were measured after 12-hours fasting. Association between shift work and biochemical variables and blood pressure were measured. The X2 and fisher's exact test was used for comparing the qualitative variables and for quantitative variables with normal distribution we used the parametric tests. Odds ratio (OR) with the 95% confidence interval (95% CI) was used for comparing the proportions of risk variables.Sub-populations in this study were consisting of 158 (37.3%) shift workers and 266 (62.7%) day workers. High levels of total cholesterol (> 200 mg/dl) and LDL-cholesterol (> 130 mg/dl) were significantly more prevalent in nearly all groups of shift workers irrespective of age. But there is no differences in the serum levels of triglyceride, HDL-C, fasting blood glucose and blood pressure between shift workers and day workers.Adjusted Odd's ratio for the effect of shift working on high serum total cholesterol and LDL-C level were 2.11(95%CI: 1.33-3.36) and 1.76(95%CI: 1.09-2.83), respectively.

CONCLUSION

This study showed that high serum total cholesterol and LDL-C level were more common in shift workers than in day workers. This finding persisted after adjustment was made for age and food type. But there was no difference in the prevalence of HDL-C, triglyceride, fasting blood glucose and hypertension between shift working and day working. It was concluded that shift work is a risk factor for lipid profile disturbances.

Individual reference intervals of hematological and serum biochemical parameters in cynomolgus monkeys

Cynomolgus monkeys, one of a number of primates phylogenetically close to humans, are commonly used in animal studies. The purpose of this study was to assess biological variations in hematological and serum biochemical parameters in cynomolgus monkeys. Summary statistics and reference intervals were calculated using data from 95 male and 95 female Chinese-bred cynomolgus monkeys aged 3 to 7 years showing no abnormalities during the breeding period. Within- and between-animal variations were estimated using a random-effect analysis of variance (ANOVA), then, a simple method that applies prior information was proposed to estimate individual reference intervals. Parameters including MCV, MCH, PT, ALP, total cholesterol, and creatinine appeared to show a large between-animal variation; thus, it is considered that individual reference intervals for these parameters would be relatively small in comparison with overall reference intervals.

Apolipoprotein E serum concentration and polymorphism in six European countries: the ApoEurope Project

As part of the ApoEurope Project, the apolipoprotein E (apo E) serum concentration and polymorphism were determined in 6934 healthy subjects aged 25-64 years recruited in six European countries: Finland; France; Greece; Northern Ireland; Portugal and Spain. Age and sex influenced apo E concentration with concentrations being significantly higher in men than in women for those aged between 25 and 44 years. The age effect differed between the sexes after the age of 44 years, displaying a linear increase in women and a plateau in men. As expected, the serum apo E concentration was highest in varepsilon2 carriers and lowest in varepsilon4 carriers in each country with a significantly higher frequency of the varepsilon4 allele in the northern regions. The main finding of this study was a clear increasing North-South gradient in serum apo E concentration independent of age, sex and apo E genotype. In subjects aged <45 years and with the varepsilon3/varepsilon3 genotype, apo E concentration was higher in the South-East (Greece) as compared to the North by 20% for men and 32% for women. In addition to the genetic polymorphism, the geographical area is an important factor to take into account when studying serum apo E concentration in multicentre studies and defining reference values.

HDL-Cholesterol, total cholesterol, and the risk of stroke in middle-aged British men

BACKGROUND AND PURPOSE

The purpose of this study was to examine the relation between serum HDL cholesterol and total cholesterol and risk of stroke.

METHODS

We carried out a prospective study in 7735 men, 40 to 59 years of age, drawn from 1 group practice in each of 24 British towns. Men with history of stroke were excluded (n=52).

RESULTS

During the mean follow-up period of 16.8 years, there were 343 stroke cases (fatal and nonfatal) in the 7683 men with no history of stroke. Higher levels of HDL cholesterol were associated with a significant decrease in risk of stroke even after adjustment for potential confounders (top fifth versus lowest fifth: adjusted relative risk=0.68, 95% CI 0.46 to 0.99). The inverse relation was seen only for nonfatal strokes (adjusted relative risk=0.59, 95% CI 0.39 to 0.90; top fifth versus lowest fifth). Total cholesterol showed no graded association with fatal strokes, but men with levels > or =8.1 mmol/L (top 5% of the distribution) showed increased risk of nonfatal stroke, although this was not statistically significant after adjustment (adjusted RR=1.46, 95% CI 0.91 to 2.32). The beneficial effects of elevated HDL cholesterol on nonfatal stroke were seen in both smokers and nonsmokers and were more evident in men with hypertension than in normotensives. In hypertensive men, elevated HDL cholesterol (top fifth) was associated with a significant 50% reduction in risk of nonfatal strokes compared with men in the lowest fifth.

CONCLUSIONS

Higher levels of HDL cholesterol were associated with a significant decrease in risk of nonfatal stroke. In contrast, elevated total cholesterol showed a weak positive association with nonfatal strokes. The marked inverse association between HDL cholesterol and stroke seen in hypertensives emphasizes the importance of those modifiable risk factors for stroke known to lower the concentrations of HDL cholesterol.

Within- and Between-Subject Variation in Commonly Measured Anthropometric and Biochemical Variables

Background: The biological variation of some commonly assessed metabolic variables in healthy subjects has not been studied extensively. The aim of the study was to assess, in 12 healthy subjects (6 male and 6 female; mean (SD) age; 22.7 (1.5) years) following an overnight fast, the day-to-day variation of body fat (impedance method), triglycerides, nonesterified fatty acid (NEFAs), glycerol, 3-hydroxybutyrate (3-OHB), lactate, glucose, insulin (RIA), C-peptide, and glucagon on 12 consecutive days.

Methods: Between- and within-subject coefficients of variation (CVG and CVW) were estimated using a random effects analysis of variance, and assay variation was subtracted to give the coefficient of within-subject biological variation (CVI). Individuality indices were calculated as CVW/CVG.

Results: The overall means, CVI, and individuality indices were as follows: for body fat, 24.2%, 10%, and 0.3; for triglycerides, 0.61 mmol/L, 21%, and 1.1; for NEFAs, 376 μmol/L, 45%, and 1.4; for glycerol, 48 μmol/L, 36%, and 0.8; for 3-OHB, 43 μmol/L, 61%, and 1.5; for lactate, 0.88 mmol/L, 31%, and 1.1; for glucose, 4.9 mmol/L, 4.8%, and 0.7; for insulin, 52 pmol/L, 26%, and 1.0; for C-peptide, 0.39 nmol/L, 24%, and 0.9; and for glucagon, 53 ng/L, 19%, and 0.8.

Conclusions: The data presented here are necessary for the evaluation of several important metabolic variables in individual and group studies. The biological variation of some metabolites makes it difficult to characterize the status of healthy subjects with a single measurement.